New insights into phase transformations in single crystal silicon by controlled cyclic nanoindentation Hu Huang and Jiwang Yan ⇑ Department of Mechanical Engineering, Keio University, Yokohama 223-8522, Japan Received 13 January 2015; accepted 1 February 2015 Available online 20 February 2015 Phase transformations in single crystal silicon were investigated by cyclic nanoindentation with controlled residual loads in the unloading process. Different phase transformations were observed at different residual loads, leading to appearance of pop-outs in different positions, which has never been reported before. Phase transformation mechanism in the reloading process was discussed by analyzing the slope change in the indentation dis- placement–time curve. Ó 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Nanoindentation; Silicon; Phase transformations; Raman spectroscopy As single crystal silicon is an important semicon- ductor material in scientific research and industrial applica- tions, its mechanical properties have been a research focus for many years. Especially, interests have been concentrat- ed in phase transformations occurring under high pressure loading and the subsequent pressure release [1–7], which affect its mechanical [8], electrical [9,10] and chemical per- formances [11]. Diamond anvil cell [1,3,5] and nanoinden- tation [2,4,6,12] are two commonly used methods to study high pressure phase transformations in single crystal silicon under a contact load. Taking advantages of high measuring resolution, small testing volume (nondestructive testing), easy to use, and good compatibility of sample size, nanoin- dentation has been given increasing attentions [13–17]. By combining Raman spectroscopy [16–20], transmis- sion electron microscopy [21], in situ electrical characteriza- tion [9,10,22,23] as well as diamond anvil cell experiments, a few phase transformation mechanisms in single crystal silicon during nanoindentation have been clarified. In the loading process, crystalline silicon (c-Si) with the diamond cubic Si-I phase transforms into a much denser metallic Si- II phase (b-Sn phase) at a pressure of 11 GPa. This trans- formation involves volume decrease, and may lead to dis- continuities in the loading curve. In the unloading processes, phase transformations of silicon depend on the unloading conditions. For fast unloading, the Si-II phase is ready to transform gradually into amorphous silicon (a-Si), leading to obvious slope change and appearance of an elbow. For slow unloading, the Si-II phase prefers to suddenly transform into high-pressure crystalline phases Si-XII/III, leading to an obvious discontinuity in the unloading curve, namely, pop-out. Although many studies have been reported on phase transformations both in single-cycle nanoindentation [14,17,24] and multi-cycle nanoindentation [25,26], the mechanisms and the paths of phase transformation are still poorly understood. There are still contradictions in conclu- sions. For example, due to the lack of direct evidence, some authors suggest that the Si-XII phase can transform into Si- II in the reloading process [27,28], but some others com- ment that the Si-XII phase is relatively stable and is hard to retransform into Si-II in reloading at the same indenta- tion load [22,25,26]. Further investigations on nanoinden- tation induced phase transformations in single crystal silicon are necessary. In this paper, a cyclic nanoindenta- tion protocol is introduced by controlling the residual load in the unloading process to obtain different initial phases for the next nanoindentation cycle. In this way, effects of different initial phases on phase transformation behaviors in the subsequent nanoindentation cycles can be investigated. The single crystal silicon (1 0 0) sample used in this study was n-type boron doped with a resistivity of 2.0  8.0 Xcm. Nanoindentation tests were performed on the ENT-1100 nanoindentation instrument (Elionix Inc., Japan) with a Berkovich indenter. For all nanoindentation tests, the max- imum indentation load was the same, 50 mN. Firstly, single- cycle nanoindentation tests with a loading/unloading rate of 5 mN/s were made on twenty points to obtain the load range for pop-out occurrence in the unloading process. Obvious pop-outs appeared in 17 nanoindentation tests, and the load range for pop-out was 7  18 mN. Then, ten cyclic nanoindentation tests were carried out with the same http://dx.doi.org/10.1016/j.scriptamat.2015.02.008 1359-6462/Ó 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. ⇑ Corresponding author; e-mail: yan@mech.keio.ac.jp Available online at www.sciencedirect.com ScienceDirect Scripta Materialia 102 (2015) 35–38 www.elsevier.com/locate/scriptamat